Charging system for mild hybrid vehicle

ABSTRACT

A charging system for a hybrid vehicle which prevents a super capacitor from being reverse charged. In the illustrative charging system a DC-DC converter is connected to the inverter and configured to receive the DC electricity from the inverter and drop voltage. A battery is configured to receive the DC electricity from the DC-DC converter and to be charged by the DC electricity. Finally, a means for preventing reverse charging is mounted on a path between the super capacitor and the battery and is configured to prevent energy from flowing from the battery to the super capacitor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation application of U.S. patentapplication Ser. No. 13/227,886, filed Sep. 8, 2011, which claimspriority to and the benefit of Korean Patent Application No.10-2010-0123826 filed in the Korean Intellectual Property Office on Dec.6, 2010, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a charging system for a mild hybridvehicle. More particularly, the present invention relates to a chargingsystem for a mild hybrid vehicle which prevents a super capacitor frombeing reverse charged quickly by voltage of a battery in a case that acharging voltage of the super capacitor is lower than that of thebattery.

(b) Description of the Related Art

Recently, environmentally-friendly vehicles such as hybrid vehicles andelectric vehicles have attracted increased attention due to energydepletion and environmental pollution. Since hybrid vehicles have anengine as power source, hybrid vehicles do not need to charge a batteryby using exterior commercial electricity. Since an electric vehicle, onthe contrary, does not have the engine, the electric vehicle must chargethe battery periodically by using exterior commercial electricity. Inaddition, the hybrid vehicle is largely classified into a mild hybridvehicles and plug-in hybrid vehicle according to charging type. A mildhybrid vehicle charges the battery by using a portion of energygenerated at an internal combustion engine, and plug-in hybrid vehicleis a hybrid vehicle that charges the battery by receiving energy fromthe exterior commercial electricity.

Since a pure electric vehicle and the plug-in hybrid vehicle receive theenergy from exterior commercial electricity, there is a large differencebetween an input terminal voltage and an output terminal voltage.Therefore, an insulated buck type DC-DC converter using a transformershown in FIG. 9 is typically used. As shown in FIG. 9, an inputcapacitor Ci is connected to a terminal of a high-voltage battery 102,an input of a switching element portion 106 having four switchingelements Q1, Q2, Q3, and Q4 formed as full bridge is connected to thehigh-voltage battery 102, and an output of the switching element portion106 is connected to a primary terminal of the transformer 108 in theinsulated buck type DC-DC converter. A voltage of the high-voltagebattery 102 is converted into an AC voltage by alternately turning onand off two pairs Q1-Q2 and Q3-Q4 of the switching element portion 106,and the AC voltage is dropped through the transformer 108 so as to applya low voltage to a secondary coil. After that the low voltage applied tothe secondary coil of the transformer 108 is rectified, the rectifiedvoltage is smoothed through an inductor L and a capacitor Co and a DCvoltage is charged in a battery 104. A duty ratio D for controlling theinsulated buck type DC-DC converter is as follows.

$D = {\frac{V_{LOW}}{2 \times V_{HIGH}} \times \frac{N_{1}}{N_{2}}}$

Herein, V_(HIGH) is the voltage of the high-voltage battery 102, V_(LOW)is a voltage of the battery 104, N₁ is a winding number of a primarycoil, and N₂ is a winding number of the secondary coil.

Since the transformer is used in the insulated buck type DC-DCconverter, efficiency is reduced due to core loss but a high-voltageside and a low-voltage side are electrically insulated. In addition, ifa voltage of an output terminal is higher than that of an inputterminal, reverse charging does not occur.

Because the difference between an input terminal voltage and an outputterminal voltage of a charging system is small in the mild hybridvehicle, a non-insulated buck type DC-DC converter shown in FIG. 10 isused instead of using the insulated buck type DC-DC converter whichincludes the transformer.

As shown in FIG. 10, the non-insulated buck type DC-DC converter 50 isdisposed between a super capacitor 120 in which a voltage generated bythe engine is stored and a battery 122. The non-insulated buck typeDC-DC converter 50 includes a switching element 126, an inductor 132, acapacitor 134, and a free-wheeling diode 130. [Please confirm that youmeant 50 rather than 124 here because there is no reference 124 in FIG.10]

The non-insulated buck type DC-DC converter 50 calculates a duty ratiofrom a voltage V_(HIGH) of the super capacitor 120 being an input and avoltage V_(LOW) of the battery 122 so as to get a target output voltage,and duty-controls the switching element 126.

The duty-control means a method that fixes a switching frequency andcontrols turn-on ratio in a waveform of a period. The duty ratio D ofthe non-insulated buck type DC-DC converter 50 is as follows.

$D = \frac{V_{LOW}}{V_{HIGH}}$

Assuming that a minimum value of the duty ratio is represented asD_(min), an equivalent impedance of loss is represented as Z_(L), afrequency of the switching element 126 is represented as f, an outputvoltage is represented as V_(o), a pulsating output voltage isrepresented as ΔV_(o), a minimum inductance L_(min) of the inductor 132and a minimum capacitance C_(min) of the capacitor 134 used in thecircuit are as follows.

$L_{\min} = {\frac{\left( {1 - D_{\min}} \right)}{2f} \times Z_{L}}$

As known from an above equation, the inductance of the inductor 132 isinversely proportional to the switching frequency f and the capacitanceof the capacitor 134 is inversely proportional to square of theswitching frequency f. If the switching frequency f is long, theinductance of the inductor 132 and the capacitance of the capacitor 134can decrease. Therefore, size of the converter can be reduced.

Because the switching frequency is made longer so as to manufacture asmaller DC-DC converter, a metal-oxide semiconductor field transistor(MOSFET) rather than an insulated gate bipolar transistor (IGBT) iswidely used as the switching element. The MOSFET can be used in thecircuit using high switching frequency, but is not suitable for use inthe circuit using high voltage and high current. The voltage in thesuper capacitor is typically low (e.g., 15V-30V) but high current flowsthrough the super capacitor in the DC-DC converter of the mild hybridvehicle. Therefore, the circuit cannot be constructed by using only oneMOSFET. Accordingly, more than two switching elements 126 are connectedin parallel with each other so as to share current capacity as shown inFIG. 11.

The super capacitor 120 used in a charging system of the mild hybridvehicle has a self-discharge circuit such that energy charged in thesuper capacitor 120 is discharged slowly. Since the discharged supercapacitor 120 can be charged in a generating mode after the vehicle isstarted, the charging voltage of the super capacitor 120 becomes loweredgradually when the vehicle is not driven for a long period of time. Ifthe charging voltage of the super capacitor 120 falls below the voltageof the battery 122, the energy charged in the battery 122 is reversecharged to the super capacitor 120 through a body diode 128 mounted atthe switching element 126 of the non-insulated buck type DC-DC converter124.

Particularly, when a power system of the mild hybrid vehicle isassembled initially, the charging voltage of the super capacitor 120 isthe lowest voltage (e.g., 3V) and difference between the chargingvoltage of the super capacitor 120 and the voltage of the battery 122 isthe largest. Therefore, when assembling the power system initially, highcurrent flows occur. That is, if the discharged super capacitor isconnected, the voltage of the battery is lowered and the chargingvoltage of the super capacitor becomes heightened as shown in FIG. 12.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE INVENTION

The present invention has been made in an effort to provide a chargingsystem for a mild hybrid vehicle having advantages of preventing a supercapacitor from being reverse charged from a battery through a DC-DCconverter due to discharge of the super capacitor in a non-insulatedbuck type DC-DC converter that charges energy in the super capacitor byusing the battery.

A charging system for a mild hybrid vehicle according to an exemplaryembodiment of the present invention may include: an engine; anintegrated starter and generator (ISG) connected to the engine andconfigured to generate 3-phase AC electricity and/or to start theengine. An inverter is configured to convert the 3-phase AC electricitygenerated at the ISG into DC electricity and/or convert the DCelectricity into the 3-phase AC electricity and to deliver the 3-phaseAC electricity to the ISG. A super capacitor is configured to receivethe DC electricity from the inverter and be charged by the DCelectricity, and may alternatively deliver the charged DC electricity tothe inverter. Further, a DC-DC converter is connected to the inverterand configured to receive the DC electricity from the inverter and dropvoltage, accordingly. A battery is configured to receive the DCelectricity from the DC-DC converter and be charged by the DCelectricity. Additionally, the present invention also includes a meansfor preventing reverse charging which is mounted on a path between thesuper capacitor and the battery and which is configured to preventenergy from flowing from the battery to the super capacitor.

According to the first exemplary embodiment of the present invention,the DC-DC converter may include a switching element configured tointerrupt a circuit. In this case, the means for preventing reversecharging is the switching element in which a body diode is removed.Alternatively in a second exemplary embodiment of the present invention,the means for preventing reverse charging may be a circuit interruptiontransistor. The circuit interruption transistor may be an IGBT or apower transistor without a body diode. Even further, the means forpreventing reverse charging may be a relay.

According to a fourth exemplary embodiment of the present invention, themeans for preventing reverse charging may be a diode forwardly biasedfrom the super capacitor to the battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a charging system for a mild hybridvehicle according to an exemplary embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating energy flow at an enginestarting in a charging system for a mild hybrid vehicle according to anexemplary embodiment of the present invention.

FIG. 3 is a schematic diagram illustrating energy flow at a charge of abattery in a charging system for a mild hybrid vehicle according to anexemplary embodiment of the present invention.

FIG. 4 illustrates a first exemplary embodiment of a non-insulated bucktype DC-DC converter used in a charging system for a mild hybrid vehicleaccording to an exemplary embodiment of the present invention.

FIG. 5 illustrates a second exemplary embodiment of a non-insulated bucktype DC-DC converter used in a charging system for a mild hybrid vehicleaccording to an exemplary embodiment of the present invention.

FIG. 6 illustrates a third exemplary embodiment of a non-insulated bucktype DC-DC converter used in a charging system for a mild hybrid vehicleaccording to an exemplary embodiment of the present invention.

FIG. 7 illustrates a fourth exemplary embodiment of a non-insulated bucktype DC-DC converter used in a charging system for a mild hybrid vehicleaccording to an exemplary embodiment of the present invention.

FIG. 8 is a schematic diagram illustrating a position at which means forpreventing reverse charging are installed in a charging system for amild hybrid vehicle according to an exemplary embodiment of the presentinvention.

FIG. 9 is a schematic diagram of a conventional charging system using aninsulated buck type DC-DC converter.

FIG. 10 is a schematic diagram of a conventional charging system using anon-insulated buck type DC-DC converter.

FIG. 11 is a schematic diagram of another conventional charging systemusing a non-insulated buck type DC-DC converter.

FIG. 12 is a graph illustrating a battery voltage, a voltage of a supercapacitor, and a current of the super capacitor in a conventionalcharging system using a non-insulated buck type DC-DC converter in acase that reverse charging occurs.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed referring to accompanying drawings in order for a personhaving ordinary skill in the art to which said subject matter pertainsto easily carry out the present invention.

Note that it is understood that the term “vehicle” or “vehicular” orother similar term as used herein is inclusive of motor vehicles ingeneral such as passenger automobiles including sports utility vehicles(SUV), buses, trucks, various commercial vehicles, watercraft includinga variety of boats and ships, aircraft, and the like.

An exemplary embodiment of the present invention will hereinafter bedescribed in detail with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of a charging system for a mild hybridvehicle according to an exemplary embodiment of the present invention;FIG. 2 is a schematic diagram illustrating energy flow at an enginestarting in a charging system for a mild hybrid vehicle according to anexemplary embodiment of the present invention; and FIG. 3 is a schematicdiagram illustrating energy flow at a charge of a battery in a chargingsystem for a mild hybrid vehicle according to an exemplary embodiment ofthe present invention.

As shown in FIG. 1 to FIG. 3, a charging system for a mild hybridvehicle according to an exemplary embodiment of the present inventionincludes an engine 10, an integrated starter and generator (ISG) 20, aPWM inverter 30, a super capacitor 40, a DC-DC converter 50, and abattery 60. The engine 10 may be any type of engine applicable to avehicle, and for example, may be a diesel engine, a gasoline engine, ora liquid propane injection (LPI) engine. A main pulley 12 is mounted ona crankshaft of the engine 10 and rotates with the crankshaft.

The ISG 20 is connected to the engine 10 through a power deliverydevice. In this specification, a belt is exemplarily shown as the powerdelivery device, but is not limited to this. For this purpose, a subpulley 22 is rotatably mounted at the ISG 20 and is operably coupled tothe main pulley 12 through a belt 14. The ISG 20 is operated as astarting motor when the engine 10 is started and is operated as agenerator when the engine 10 runs so as to charge the battery 60. TheISG 20 generates a 3-phase AC voltage.

The pulse width modulation (PWM) inverter 30 is electrically connectedto the ISG 20 so as to receive the 3-phase AC voltage, and converts the3-phase AC voltage into a DC voltage. In addition, the PWM inverter 30converts the DC voltage stored in the super capacitor 40 into the3-phase AC voltage and delivers the 3-phase AC voltage to the ISG 20when the engine 10 is started.

A DC terminal of the PWM inverter 30 is simultaneously connected to thesuper capacitor 40 and the DC-DC converter 50. Therefore, the DC voltageis applied to the super capacitor 40 and the DC-DC converter 50 when thebattery 60 is charged.

A non-insulated buck type DC-DC converter may be a used as the DC-DCconverter 50. One side terminal of the DC-DC converter 50 iselectrically connected to the PWM inverter 30 such that a high voltageis applied thereto, and the other side terminal of the DC-DC converter50 is electrically connected to the battery 60 such that a low voltageis formed.

When the mild hybrid vehicle is started, energy stored in the supercapacitor 40 is delivered to the ISG 20 through the PWM inverter 30, asshown in FIG. 2. Then, the ISG 20 is operated as the starting motor andcranks the engine 10. At this time, the DC-DC converter 50 does notcharge the battery 60. That is, DC-DC converter 50 and the PWM inverter30 receive signal from an electric control unit (ECU) through CANcommunication and prepare the operation for starting.

If the mild hybrid vehicle is driven by the engine 10, the ISG 20generates electricity, as shown in FIG. 3. The 3-pahse AC electricitygenerated at the ISG 20 is converted into the DC electricity by the PWMinverter 30. The converted DC electricity is charged in the supercapacitor 40 and is simultaneously input to the DC-DC converter 50. TheDC-DC converter 50 calculates a duty ratio of a control signal accordingto an input voltage V_(HIGH) of DC-DC converter 50 and the voltage ofthe battery 60 and duty-controls a switching element in the DC-DCconverter 50.

Main function of the switching element in the DC-DC converter 50 isturning-on or turning off a circuit according to the control signal.Types of switching elements are different in accordance with a controlcircuit, a voltage, a current capacity, and a switching frequency, butan MOSFET or an IGBT may be used as the switching element. If theswitching element is turned-off and the circuit including an inductor isopen, the current briefly becomes zero. Therefore, very high voltage isgenerated between both ends of the inductor. Since many elementsincluding the inductor can be damaged due to the high voltage, a pathfor discharging energy inductor load should be secured even though thecircuit is open by the switching element. For this purpose, the circuitin which a body diode is connected to both ends of the switching elementis generally used. The switching element and body diode may beseparately manufactured and then be assembled, but the body diode may beconnected in parallel to the switching element when manufacturing theswitching element.

As described above, the energy charged in the battery 60 canunfortunately be reverse charged to the super capacitor 40 by the bodydiode. The present invention, however, prevents the super capacitor 40from being reverse charged according to an exemplary embodiment of thepresent invention.

FIG. 4 illustrates a first exemplary embodiment of a non-insulated bucktype DC-DC converter used in a charging system for a mild hybrid vehicleaccording to an exemplary embodiment of the present invention.

As shown in FIG. 4, the non-insulated buck type DC-DC converter 50according to the first exemplary embodiment of the present invention mayinclude a switching element 51, an inductor L 53, a capacitor C 54, anda free-wheeling diode 52. In the illustrative embodiment, the body diodeis removed from the switching element 51. That is, the energy of thebattery 60 is prevented from being delivered to the super capacitor 40through the body diode. Therefore, the reverse charging of the supercapacitor 40 may be prevented. Since the other constituent elements ofthe DC-DC converter 50 are well-known to a person of an ordinary skillin the art, detailed description thereof will be omitted in thisspecification.

Typically a switching element 51 (e.g., the MOSFET and the IGBT)includes a body diode. The body diode may be formed incidentally inmanufacturing processes of the switching element 51, but can be removedon purpose in above the processes. Since the body diode connected to theboth ends of the switching element 51 is the only element forming aforward path from the battery 60 to the super capacitor 40, theswitching element 51 should be manufactured so that the body diode isnot formed. If amount of impurities are controlled in the manufacturingprocesses, the body diode may be prevented from being formed orresistance of the body diode can be increased according to the MOSFET.

Compared to the MOSFET, the body diode can be easily removed accordingto the IGBT because removing process of the body diode is very simple.Since the switching frequency of the MOSFET is higher than that of theIGBT, the MOSFET is typically used in the circuit using high switchingfrequency

Since MOSFETs typically cannot pass high current, MOSFETs are used inthe circuit having a small current capacity compared with IGBTs. Inorder to increase current capacity, the MOSFETs are connected inparallel with each other. If the number of MOSFETs connected in parallelwith each other increases, the number of paths through which the reversecharging occurs also increases. Therefore, if the MOSFET without thebody diode is applied to the buck type DC-DC converter 50, the reversecharging from the battery 60 to the super capacitor 40 may be prevented.

The body diode in the IGBT can be removed easily compared to the MOSFET,but the switching frequency of the IGBT is lower than that of theMOSFET. Therefore, a smaller sized product cannot be manufactured. Sincethe DC-DC converter 50 used in the mild hybrid vehicle should berelatively small in size, the MOSFET using higher switching frequencyshould generally be used.

FIG. 5 illustrates a second exemplary embodiment of a non-insulated bucktype DC-DC converter used in a charging system for a mild hybrid vehicleaccording to an exemplary embodiment of the present invention.

As shown in FIG. 5, the DC-DC converter 50 according to the secondexemplary embodiment of the present invention uses a switching element51′ including the body diode 55. However, a circuit interruptiontransistor 70 without the body diode is installed between the supercapacitor 40 and the DC-DC converter 50. The IGBT or a power transistor(Power BJT Transistor) without the body diode can be used as the circuitinterruption transistor 70. In order to use the circuit interruptiontransistor 70, an electric control unit (ECU) detects the voltage of thesuper capacitor 40 and the voltage of the battery 60 and turns on thecircuit interruption transistor 70 if the voltage of the battery 60 ishigher than that of the super capacitor 40. Therefore, path though whichenergy is delivered from the battery 60 to the super capacitor 40 is cutoff and the super capacitor 40 cannot be reverse-charged.

FIG. 6 illustrates a third exemplary embodiment of a non-insulated bucktype DC-DC converter used in a charging system for a mild hybrid vehicleaccording to an exemplary embodiment of the present invention.

As shown in FIG. 6, the DC-DC converter 50 according to the thirdexemplary embodiment of the present invention uses the switching element51′ including the body diode 55. However, a relay 80 is installedbetween the super capacitor 40 and the DC-DC converter 50. In order touse the relay 80, the electric control unit (ECU) detects the voltage ofthe super capacitor 40 and the voltage of the battery 60, and turns offthe relay 80 if the voltage of the battery 60 is higher than that of thesuper capacitor 40. Therefore, a path though which energy is deliveredfrom the battery 60 to the super capacitor 40 is cut off and the supercapacitor 40 cannot be reverse-charged.

FIG. 7 illustrates a fourth exemplary embodiment of a non-insulated bucktype DC-DC converter used in a charging system for a mild hybrid vehicleaccording to an exemplary embodiment of the present invention.

As shown in FIG. 7, the DC-DC converter 50 according to the fourthexemplary embodiment of the present invention uses the switching element51′ including the body diode 55. However, a diode 90 forwardly biasedfrom the super capacitor 40 to the battery 60 is installed between thesuper capacitor 40 and the DC-DC converter 50. Current can flow from thesuper capacitor 40 to the battery 60, but cannot flow from the battery60 to the super capacitor 40 due to the diode 90. Therefore, reversecharging of the super capacitor 40 can be prevented.

FIG. 8 is a schematic diagram illustrating a position at which means forpreventing reverse charging are installed in a charging system for amild hybrid vehicle according to an exemplary embodiment of the presentinvention.

As shown in FIG. 8, means for preventing reverse charging (the switchingelement 51 without the body diode, the circuit interruption transistor70, the relay 80, and the diode 90) can be installed at any position 91,92, 93, 94, and 95 that connects the super capacitor 40 and the battery60. For example, the means for preventing reverse charging can beinstalled at positions 91 and 92 between the super capacitor 40 and theDC-DC converter 50, at a position 93 in the DC-DC converter 50, or atpositions 94 and 95 between the DC-DC converter 50 and the battery 60.

As described above, since paths forwardly bias from a battery to a to asuper capacitor is removed by using a switching element without a bodydiode, the battery is prevented from being reverse-charged through thebody diode of a DC-DC converter due to self-discharge of the supercapacitor in a mild hybrid vehicle according to some embodiment of thepresent invention.

Since the switching element without the body diode is inserted in aseries path between the super capacitor and the battery and theswitching element is turned on or off by comparing the voltage of thesuper capacitor with that of the battery, reverse charging of the supercapacitor can be prevented according to some embodiment of the presentinvention.

Since a relay is inserted in a series path between the super capacitorand the battery and the relay is turned on or off by comparing thevoltage of the super capacitor with that of the battery, reversecharging of the super capacitor can be prevented according to someembodiment of the present invention.

Since a current-blocking diode is inserted in a series path between thesuper capacitor and the battery and the current-blocking diode isreversely biased if the voltage of the super capacitor decreases,reverse charging of the super capacitor can be prevented according tosome embodiment of the present invention.

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A charging system for a hybrid electric vehicle,comprising: an engine; an integrated starter and generator (ISG)connected to the engine and configured to generate AC electricity and tostart the engine; an inverter configured to convert the AC electricitygenerated at the ISG into DC electricity, to convert the DC electricityinto the AC electricity, and to deliver the converted AC electricity tothe ISG; a super capacitor configured to receive the DC electricity fromthe inverter and be charged by the DC electricity; a DC-DC converterconnected to the inverter and the super capacitor and configured toreceive the DC electricity from the inverter or the super capacitor anddrop voltage; a battery configured to receive the DC electricity fromthe DC-DC converter and to be charged by the DC electricity; and a relaymounted on an electric path between the super capacitor and the batteryand configured to prevent energy from flowing from the battery to thesuper capacitor.
 2. The charging system of claim 1, wherein the relay ismounted on the electric path between the super capacitor and the DC-DCconverter.
 3. The charging system of claim 1, wherein the relay ismounted on the electric path in the DC-DC converter.
 4. The chargingsystem of claim 1, wherein the relay is mounted on the electric pathbetween the DC-DC converter and the battery.
 5. The charging system ofclaim 1, wherein the DC-DC converter is a non-insulated buck type DC-DCconverter.
 6. The charging system of claim 1, wherein the relay isturned off if voltage of the battery is higher than that of the supercapacitor.
 7. A charging system for a hybrid electric vehicle,comprising: an engine; an integrated starter and generator (ISG)configured to generate electricity and to start the engine; a supercapacitor configured to receive and be charged by the electricity fromthe ISG; a converter connected to the super capacitor and configured toreceive the electricity from the super capacitor and drop voltage; abattery configured to receive and be charged by the electricity from theconverter; and a relay mounted on an electric path between the supercapacitor and the battery and configured to prevent energy from flowingfrom the battery to the super capacitor.
 8. The charging system of claim7, wherein the relay is mounted on the electric path between the supercapacitor and the converter.
 9. The charging system of claim 7, whereinthe relay is mounted on the electric path in the converter.
 10. Thecharging system of claim 7, wherein the relay is mounted on the electricpath between the converter and the battery.
 11. The charging system ofclaim 7, wherein the converter receives DC electricity from the supercapacitor and delivers the dropped DC voltage to the battery.
 12. Thecharging system of claim 11, wherein the converter is a non-insulatedbuck type DC-DC converter.
 13. The charging system of claim 7, whereinthe relay is turned off if voltage of the battery is higher than that ofthe super capacitor.
 14. A system for preventing reverse charging in ahybrid electric vehicle, comprising: a super capacitor configured toreceive and be charged by the electricity from a generator; a converterconnected to a super capacitor and configured to receive electricityfrom the super capacitor and drop voltage; a battery configured toreceive and be charged by the electricity from the converter; and arelay mounted on an electric path between the super capacitor and thebattery and configured to prevent energy from flowing from the batteryto the super capacitor.
 15. The charging system of claim 14, wherein therelay is mounted on the electric path between the super capacitor andthe converter.
 16. The charging system of claim 14, wherein the relay ismounted on the electric path in the converter.
 17. The charging systemof claim 14, wherein the relay is mounted on the electric path betweenthe converter and the battery.
 18. The charging system of claim 14,wherein the converter receives DC electricity from the super capacitorand delivers the dropped DC voltage to the battery.
 19. The chargingsystem of claim 18, wherein the converter is a non-insulated buck typeDC-DC converter.
 20. The charging system of claim 14, wherein the relayis turned off if voltage of the battery is higher than that of the supercapacitor.